turnratio

Turn ratio, in the context of electrical transformers, is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. It is typically expressed as Np/Ns, where Np is the primary turns andNs is the secondary turns. The turn ratio determines how voltages and currents are transformed between windings and is often denoted by a.

In an ideal transformer, the turn ratio a equals Np/Ns and governs the voltage and current relationships: Vp/Vs = Np/Ns = a, so Vs = Vp / a. The currents relate by Ip/Is = Ns/Np = 1/a, so Is = a Ip. Power is conserved in the ideal case, with Vp Ip ≈ Vs Is.

Impedance reflection is another key consequence: an impedance connected to the secondary appears on the primary as Z_in = a^2 Z_load, where a = Np/Ns. This means the turns ratio affects how external loads are seen by the source and influences input impedance, voltage regulation, and efficiency.

Non-ideal factors such as winding resistance, leakage inductance, and magnetizing current cause deviations from the ideal relationships. Real transformers exhibit voltage regulation, losses, and a slight change in the effective turns ratio under load. Measurement of the turns ratio is commonly performed with transformer turns ratio testers. The turn ratio is fundamental in selecting transformers for stepping voltage up or down, matching impedances, and ensuring proper operation in power distribution, power supplies, and signal coupling. In autotransformers, a portion of the winding is common to both sides, which alters the effective turn ratio and voltage transfer characteristics compared with two-winding transformers.